Polyurethane (PU) polymers are known in the coating industry to be useful as a part of polymeric coatings because they have excellent resistance to abrasion, chemicals and solvents. These polymers can be designed to be highly flexible and very durable. Conventional oil modified polyurethane polymers are prepared in organic solvents such as aliphatic hydrocarbons and applied as clear or pigmented coatings. After application, the solvent is allowed to evaporate and the film cures via crosslinking, by air oxidation of the ethylenic groups attached to the polyurethane backbone. A principal application for these coatings has been as clear coatings for wood flooring. Coating compositions prepared with higher levels of solids will release less hydrocarbon solvent than compositions that have higher amounts of volatile organic compound (VOC) levels.
In an effort to reduce the amount of VOCs released, and maintain performance, manufacturers have been using water dispersible polyurethanes. For example, water-based anionically stabilized polyurethane polymers are prepared by reacting polyols and dihydroxy carboxylic acid compounds with an excess of diisocyanate to provide a carboxy functional prepolymer having NCO terminal groups. The acid groups are neutralized with tertiary amines to provide salt groups. The neutralized prepolymer can be readily dispersed in water. The terminal NCO groups can be reacted with compounds having active hydrogen groups, wherein the active hydrogen groups are more reactive than water, to produce a fully reacted polyurethane polymer. Typically, to facilitate manufacture, the prepolymer is made in the presence of a solvent that is either left in the dispersion, or removed as one of the last steps in production to provide a low solvent or solvent-free product. Solvents that can be easily removed are usually volatile solvents, such as acetone or methyl ethyl ketone. Alternatively, the anionic stabilizing group of the water dispersible polyurethane polymers can be replaced with cationic stabilizing groups or non-ionic stabilizing groups, to facilitate water dispersibility. Generally, the polymers are linear and their films vary from hard and relatively inflexible to soft and highly flexible. The applications for these water-borne urethane polymers include clear and pigmented coatings for concrete, metal, wood, semi-rigid and flexible plastics, rubber, leather, glass fiber sizing, printing inks and adhesives.
The water-based polyurethane polymers are usually linear polymers. These polymers produce films that have chemical resistance lower than the highly crosslinked films of two component solvent-borne urethane systems. Preparing the polyurethane coating compositions by pre-crosslinking a water-based polyurethane polymer can be accomplished by incorporating a large amount of monomers(s) that have more than two reactive functional groups in the prepolymer stage. However, this results in highly viscous prepolymer blends that cannot be easily dispersed in water. Products using a high percentage of pre-crosslinking of the prepolymer at the dispersion stage, or using chain extenders having more than two reactive functional groups, can result in significant gelling of the polymer.